1. Field of the Invention
This invention relates to a liquid crystal projection apparatus for extensible projecting an image displayed mainly on a small-sized liquid crystal panel and to a liquid crystal panel mainly using in said liquid crystal projection apparatus.
2. Description of the Prior Art
Research and developmental activities have been accelerated on liquid crystal display apparatus because they offer a large number of advantages such as light weight, thin structure and the like. On the other hand, however, they are disadvantageously difficult to be made into a large display area. Under such a circumstance, a liquid crystal projection type television in which a displayed image of a small-sized liquid crystal panel is extensible projected with a projection lens to obtain a large picture thereof has suddenly been watched with keen interest recently. A liquid crystal projection type television now being sold uses a twisted pneumatic (hereinafter called a TN) liquid crystal panel in which a rotary polarization characteristic of the liquid crystal is utilized.
First, an explanation follows on a general liquid crystal panel. A conventional TN liquid crystal panel uses two polarizers. The operation thereof will be explained below.
An incident light is passed through and incident side polarizer to become an undirectionally polarized light and enters into a liquid crystal panel. When the liquid crystal is in the OFF condition, the incident light is rotated by 90 degrees, and when the liquid crystal panel is in the ON condition, it is transmitted therethrough with no rotation. As a result, if the polarizers on the incident side and exit side are orthogonal in polarization direction, when the panel is in the OFF condition, the light will be transmitted, and if the panel is in the ON condition, it will be stopped. However, if the polarization directions of the two polarizers are parallel to each other, the inverse phenomena can be obtained. As shown above, the conventional liquid crystal panel modulates a light to display an image.
Next, a conventional liquid crystal projection apparatus will be explained below, which comprises a light omitting means such as a light condensing optical system or the like, an infrared cut mirror for transmitting the infrared rays, a blue light reflecting dichroic mirror (hereinafter called a BDM), a green light reflecting dichroic mirror (hereinafter called a GDM), a red light reflecting dichroic mirror (hereinafter called a RDM), an optical component such as a projection lens system or the like, three sets of incident and exit side polarizers disposed correspondingly to the blue, green and red light, and a conventional TN liquid crystal panel. With the apparatus composed as above, the operation will be explained below.
First, a white light emitted from the light condensing optical system has a blue light (hereinafter called a B-light) reflected by the BDM and sent to the polarizer. The light that has passed through the BDM has a green light (hereinafter called a G-light) reflected by the GDM, and a red light (hereinafter called a R-light) reflected by the RDM and sent to the corresponding polarizers. The polarizers each transmit only one light of the longitudinal wave component and the transversal wave component of the corresponding one of the blue, green and red lights to align the direction of polarization with each other and sent to the corresponding liquid crystal panel. In this case, more than 50% of the light will be absorbed by the polarizers, which means that the brightness of the transmitted light will be below one half of the maximum value.
Each liquid crystal panel modulates the transmitted light in response to a video signal. The modulated light is passed through the corresponding polarizer according to the modulation degree and sent to the corresponding projection lens system for extensible projecting on the projection screen.
As is clear from the above explanations, a liquid crystal panel using a TN liquid crystal requires a linearly polarized light incident to the liquid crystal panel. Accordingly, polarizers must be disposed on the front and back sides of the liquid crystal panel. The above-described polarizers absorb more than 60% of the light on a theoretical basis. As a result, there arises such a problem in that when extensible projected on the projection screen, only a low luminance picture can be obtained.
As a liquid crystal panel using no polarizer and a liquid crystal projection apparatus using such a panel, an element having a pneumatic liquid crystal and a diffraction grating in combination is proposed in U.S. Pat. No. 4,389,096 or the like; however, it is difficult to align the liquid crystal molecules if an irregularity such as that caused by the diffraction grating having a small pitch as well as having a height is formed on the surface of a substrate.
In order to overcome these problems, this invention uses a polymer dispersed liquid crystal. A liquid crystal panel using the polymer dispersed liquid crystal has a much improved light application efficiency because no polarizer is needed. Because of no need to control alignment, even if the substrate has an irregularity caused by a diffraction grating, no problem arises.
A brief description follows on a polymer dispersed liquid crystal. The polymer dispersed liquid crystal can be broadly classified into two types depending on the liquid crystal and the dispersed condition thereof. One is a type that droplet-shaped liquid crystal particles are dispersed into a polymer, that is, the liquid crystal exists the polymer in a discontinuous condition. This is hereinafter called a PDLC, and a liquid crystal panel using such a type of liquid crystal is called a PD liquid crystal panel hereinafter. The other PD crystal of a type that a liquid crystal layer has a network-like polymer, structure resembling a sponge having liquid crystal particles dispersibly contained, therein, which means that the liquid crystal exists therein not in a droplet-shaped state but in a continuous state. This is hereinafter called a PNLC, and a liquid crystal panel using such a type of liquid crystal is called a PN liquid crystal panel hereinafter. An image using the PD and PN types of liquid crystal panels, by controlling a light to be scattered and transmitted.
The PD liquid crystal panel uses a property that the refractive index is different depending on the alignment direction of liquid crystal molecules. When a voltage is not applied thereto, the droplet-shaped liquid crystal molecules are aligned in irregular directions, under which, the polymer and liquid crystal are different in refractive index from each other, so that the incident light is scattered. Here, if the voltage is applied, the liquid crystal molecules are aligned in the same direction. As a result, if the refractive index of liquid crystal whose molecules are aligned in one specific direction is made equal to that of the polymer, the incident light can pass therethrough without being scattered.
Contrary to this, the PN liquid crystal uses the irregularity of the liquid crystal molecular alignment itself. Under the irregular alignment state, that is, under the application of no voltage, the incident light will be scattered. On the other hand, if a voltage is applied to make the molecular alignment regular, the light can pass therethrough. In this case, however, the explanations on the movements of liquid crystal molecules of the PD and PN liquid crystal panels are made only as one model. This invention is not limited to either a PD or PN liquid crystal panel, but the explanations will be made typically on the PD liquid crystal panel for simplification. The PD and Pn liquid crystal panels are generically called polymer dispersed liquid crystal panels. Also, the liquids containing a liquid crystal to be injected into a polymer dispersed liquid crystal panel are generically called liquid crystal solutions or resins, and the state that the resin constituent of the liquid crystal solution is polymerization-cured is called a polymer.
As a polymer matrix of a liquid crystal layer of such a polymer dispersed type of liquid crystal display device, either thermoplastic or thermosetting resin may be used basically as long as it is transparent. However, an ultraviolet curing resin is generally used in many cases because of being very simple and highly performable. This is because the production method of a conventional TN mode liquid crystal panel can be directly applied for this purpose. The production method of a convention liquid crystal panel is that a predetermined electrode pattern is formed on each of the upper and lower substrates and these tow substrates are superposed so that the electrodes thus patterned thereon confront each other. In this case, spacers each having a predetermined particle size are disposed between the two substrates, and these substrates are fixed with a sealing material of epoxy resin to hold a gap therebetween. Next, a liquid crystal is injected into the gap thus held, thus producing a liquid crystal panel.
In order to product a polymer dispersed type of liquid crystal panel applying this production method, if an ultraviolet curing resin, for example, an acrylic resin is used as polymer matrix material, is exists before injection as a comparatively low viscosity precursor such as a monomer or polymer, so that a material blended with the liquid crystal is fluid enough to be injected at room temperature. As a result, using the convention liquid crystal panel production method, a liquid crystal panel of the polymer dispersed type can be easily fabricated by irradiating light after injection to promote the curing reaction, thereby forming a polymer dispersed liquid crystal layer.
Besides, by applying ultra-violet irradiation onto the panel after injection, the polymerization reaction occurs only with the resin to become a polymer, and thus, only the liquid crystal is subjected to phase separation. If the dispersed amount of liquid crystal is smaller than the resin, droplet-shaped liquid crystal particles are formed so as to be isolated from each other. On the other hand, if the dispersed amount of liquid crystal is larger than the resin, the polymer matrix exists in the liquid crystal material in a granular or network pattern, and thus, the liquid crystal forms a continuous layer. In this case, unless the particle six of droplet-shaped liquid crystal particles or the hole size of polymer network is approximately uniform and yet ranges from 0.1 to several microns, the light has inferior scattering characteristics, so that the contrast ratio cannot be improved. In order to overcome these problems, a material that can be cured in a comparatively short period of time must be used, so that is preferable to use a resin of the ultra-violet curing type.
The operation of the polymer dispersed liquid crystal panel will be briefly explained below. When a voltage is not applied, the liquid crystal molecular direction of each liquid crystal droplet becomes irregular, under which, there occurs a difference in refractive index between the polymer and the liquid crystal, resulting in the scattering of the incident light. Here, if the voltage is applied to the electrodes, the liquid crystal molecular directions are aligned with each other. As a result, by making the refractive index of the liquid crystal when the molecules are aligned with each other in a specific direction equal to the refractive index of the polymer in advance, the incident light can be transmitted without being scattered.
As explained above, because of no need to use of polarizer, the polymer dispersed liquid crystal panel makes it possible to improve the light application efficiency as well as to obtain a displayed image with an extremely high luminance. However, the following problems occur if the above-mentions liquid crystal is intended to be used for a liquid crystal panel:
One is the delamination of the polymer dispersed liquid crystal layer and the confronted electrodes or pixel electrodes. This is due to the fact that adhesion between the electrode composed of ITO or the like and the polymer dispersed liquid crystal layer is disadvantageously low. With the liquid crystal projection type television, when a lamp as a light source is turned on, a temperature of 50.degree. to 60.degree. C. is applied to the liquid crystal panel, and when it is turned OFF, it becomes 10.degree. to 30.degree. C. in temperature. This means that the liquid panel of a liquid crystal projection television is unavoidably subjected to such a severe environmental condition that a heat shock test is carried out for every cycle of the power source being turned ON and OFF. This is one reason for delamination.
The other is an inferior scattering characteristic. In order to practically use the polymer dispersed liquid crystal panel as a device in a liquid crystal projection type television, it is required that it can be driven at a low voltage as well as having a sufficient contrast ratio. Especially, in order to obtain a good display characteristic, it is preferable that the direct view type panel have a contrast ratio of 30:1 or more, and the projection type panel have a contrast ratio of 100:1 or more.
In order to increase the contrast ratio, it is necessary to improve the scattering characteristic. Though perfect diffusion can be considered as one target of the scattering action, the contrast ratio CR in the perfect diffusion can be calculated as EQU CR=1/sin.sup.2, .sigma.
(Dewey, Proc. of SID, p. 138, 1977), where .sigma. is a converging angle (half-angle). In order to increase the contrast ratio, it is necessary to improve the scattering characteristic. The scattering characteristic can be improved by increasing the thickness of the polymer dispersed liquid crystal layer, but because the driving voltage is increased, there arises a problem in that the TFT drive becomes difficult. At present, the light scattering characteristic of the polymer dispersed liquid crystal panel has not yet been attained to the perfect diffusion-condition as an ideal scattering characteristic condition.
Especially, when used as the projection type display, the F-number of the concave mirror condensing optical system using a metal halide lamp which is now being used generally and the projection optical system matched therewith ranges from 4 to 5, so that the contrast ratio obtained when a presently known polymer dispersed liquid crystal panel is used is unsatisfactorily about 50:1.